Here a group of students are on tour to gains experience on Nordic skating. The ice looks goog and is good in most places, while in other areas the worm of skaters take turns to avoid the weakest spots by following the leader.
Finding skatable ice is often a larger effort, unless you are prepared to depend on reports from other people and skate on consumed ice. Weather reports, forecasts and observations from previous weeks, shipping ice reports, wind directions, satellite images and experience are part of the sources to study before you decide on where to look for good ice. After possibly good ice has been located by desk work a tour leader would go and observe the ice before taking a group out. It may require travelling, skiing or using ice roads. In the best case scenario one is ok with going to the closest shore to take a spin of tens of kilometers.
Ice thickness varies a lot. This video demonstrates ice which is only a couple of cm thick. Jöns carries a boyancy rucksack, and is connected with a throw line to the shore with which to pull back in case he floats and has no other method of turning back.
Perfect skating ice usually does not last for long. A snow fall makes evaluating ice quality harder. The video was filmed on a section where waves formed along the Gulf of Finland and the Baltic Sea starts to break ice. The outer edge will break first, forming a barrier of crush and possibly pack ice. A bit further in waves still travel, but under the ice that bends unless the waves are steep enough to break it, initially to large skateable sheets of ice that as waves continue, break into even smaller sheets. Here the waves limited the skating area most.